1. Field of the Invention
The present invention relates to a process and apparatus for determining whether a photomask for making semiconductor devices is good or defective.
2. Background Art
A high quality photomask is necessary for the art of transferring a circuit pattern to semiconductor wafers for use in a process of making semiconductor integrated circuits. The production of a photomask includes filming a glass substrate with chrome, coating the filmed substrate with resist to make a mask blank, and describing or forming a mask pattern in the blank.
The position on a semiconductor wafer to which the mask pattern in a photomask is transferred is determined by the position of the pattern relative to the wafer. It is therefore essential for the pattern to be formed at a correct position in the mask blank.
It is therefore necessary to position a mask blank correctly relative to the position where a mask pattern should be formed in an apparatus for forming mask patterns to make photomasks. This requires the blank to be positioned correctly on a blank holder. In practical processes, however, mask blanks may not necessarily be positioned correctly. It is therefore necessary to determine whether a mask pattern is formed correctly on a mask blank. In order to confirm the position of the pattern on the blank, a coordinate measuring machine using a laser interferometer may be used. In general, however, such a coordinate measuring machine is expensive.